Among complicated timepieces, striking watches are particularly popular. However, their small volume often limits sound emission, and the user has to pay particular attention to hear the acoustic message properly.
There are known specific watch supports acting as a sound box, on which the user places his watch when he wishes to hear the sound produced by the watch better, and which do not fall within the scope of the present invention.
There are known cases or case middles including resonant chambers on which are mounted the sound producing organs, gongs or vibration plates, as in FR Patent Application No 2777095A1 in the name of CHRISTOPHE CLARET SA wherein, in some variants, membranes are placed in contact with walls of such resonant chambers, which take up a large part of the available internal volume inside the case or case middle.
EP Patent No 2034376B1 in the name of ROTH ET GENTA SA attempts to overcome this problem of space, by proposing a thin case middle as a resonator, including lugs for securing gongs or suchlike.
CH Patent Application No 645236G in the name of BOUILLE discloses a case including peripheral lugs intended for mechanical fasteners: due to this specific function, they have increased mechanical rigidity. The geometric shapes, fastenings and orientation of such lugs are well defined and acoustically inefficient, and they could not serve as external acoustic resonators.
There are also known acoustic resonators where sounds are produced by a membrane which is caused to vibrate by an elastic strip, and whose vibrations are maintained by an electromagnetic transducer, as in CH Patent Nos 497760 and 497017, in the name of SPADINI, which have substantial dimensions and require an electric circuit.
To improve the sound level of a striking watch, it is necessary to optimise the elements that form the external parts of the watch, so that they radiate the frequencies generated by the striking system, i.e. in general the natural frequencies of the strips of a vibration plate, or the natural frequencies of a gong or minute repeater. To obtain a rich sound (in the case of a minute repeater) or a sound of uniform intensity regardless of the notes played (in the case of a musical box watch), the external parts must be able to respond to a broad range of frequencies between 1 kHz and 6 kHz, which are the frequencies at which perception by the human ear is optimal.
However, because of their size, shape and rigidity, the natural modes of the external parts of a watch are mainly at a high frequency. In such case, the external parts filter rather than radiate the frequencies of the strike sound or melody.
It is known to optimise the external parts so that they radiate a single given frequency within the frequency band concerned (1 kHz-6 kHz), as in FR Patent Application No 1136675A in the name of Gebrueder JUNGHANS A. G., which discloses the use of a standard membrane for a striking clock, or so that the external parts radiate several modes within the frequency band concerned (1 kHz-6 kHz), as in EP Patent Application No 2461219A1 in the name of MONTRES BREGUET SA disclosing the use of an optimised spatially in homogeneous membrane, or also in EP Patent No 2367079B1 in the name of MONTRES BREGUET SA, which discloses a solution wherein the watch crystal forms the vibrating, radiating member, owing to a particular arrangement for securing the crystal to the bezel.
In the state of the art, and for conventional watch designs, it is, however, impossible to obtain acoustic resonance of the external parts at a frequency of less than 1500 Hz, because the main function of the external components is to ensure protection of the movement, and generally, the sealing of the watch, and they must therefore be sufficiently rigid and thick. For the same reason, it is impossible to greatly increase the number of acoustic resonances of the external parts within the audible frequency range concerned (from 1 kHz to 6 kHz), which considerably limits the sound level and the richness of the sounds emitted and melodies played.